The invention relates to apparatus for controlling the lapping and polishing of plan parallel wafers to close thickness tolerance. More specifically it relates to apparatus for reliable and accurate automatic lapping control and to improvements of conventional lapping control apparatus. One major application is the lapping and polishing of piezoelectric materials such as ceramic or quartz crystal wafers intended for frequency control applications and requiring precise thickness control. Another application is lapping and polishing of nonpiezoelectric materials.
There are various types of conventional machines used for lapping flat wafers. Two examples are the planetary lap and the excentric or pin lap. In both machines the wafers are positioned between two lapping plates and moved with respect to the latter by means of socalled carriers. These are made of sheets of material thinner than the wafers and contain cutouts for the wafers. A lapping slurry, usually consisting of a water or oil based suspension of grinding powder, such as carborundum or aluminum oxide, is fed between the lapping plates and serves to grind and flush away the wafer particles. For polishing, a finer powder is used, and the plates may be covered by a buffeting surface. In another type of lapping machine, the wafers are again located between two plates but fixed in position--for example by waxing--to the surface of one plate. The two plates are moved relative to each other, and a slurry is fed between them. The wafers are lapped one side at a time.
The planetary lapping machine is explained in more detail below in conjunction with the description of the invention.
The main conventional methods for controlling the lapping process are described below and referred to as Methods 1 through 5.
Method 1 is based on an empirical relationship between lapping speed and lapping time. Lapping is terminated after a specified time at a constant speed.
Method 2 is based on monitoring the wafer thickness by means of measuring the distance between the lapping plates. This distance can be related to the width of an air gap between two surfaces that are referenced to the two respective lapping surfaces. The gap can be measured by various means such as air gauges or capacitive measurements.
Method 3 is based on mechanical stops that serve to limit the thickness of the lapping load from decreasing below a preset value. One approach is to use spacers between the lapping plates made from hard material such as diamond. Another approach uses the carriers such as the spacers.
Methods 1, 2, 3 are simple but relatively inaccurate. In Method 1 the accuracy can be improved by repeated unloading, measuring, reloading and relapping of the wafers. In Methods 2 and 3 the thickness is controllable to a tolerance of about .+-.0.005 mm, which is insufficient for precision applications such as the lapping of thin quartz wafers. An advantage of Methods 1, 2 and 3 is that they can be easily automated.
Methods 4 and 5 are used for lapping wafers consisting of piezoelectric material. They are based on the piezoelectric effect which causes a piezoelectric wafer to vibrate mechanically when exposed to an a. c. signal, and to emit an a. c. signal when exposed to mechanical vibrations. In a lapping machine the mechanical vibrations are exerted on the wafer by the grinding action of slurry and lapping plates, and the corresponding a. c. signals appear between the lapping plates. The frequency of these signals corresponds to the resonance frequencies of the wafers and is therefore related to their dimensions. For example, in flat AT cut quartz wafers the resonance frequency is related to the thickness by approximately EQU f=1.66.times.10.sup.6 /T (1)
where f is measured in Hz and T is the wafer thickness in mm. Hence during lapping the wafer frequency increases inversely proportional to T. For example, at a frequency of 32.2 MHz, the wafer thickness is 0.05 mm according to (1). Lapping the polishing of flat AT cut quartz wafers is routinely done up to about 35 MHz and is feasible to above 60 MHz. Desired thickness control is on the order of .+-.0.1%, which for the above example corresponds to a thickness tolerance of .+-.0.00005 mm.
In Method 4 a radio receiver or similar frequency selective sensor is connected to the lapping plates to monitor the signals emitted by the wafers as they are being lapped. Normally the resonance frequencies of the individual wafers are different from each other and extend over a frequency "spread" between the lowest and highest wafer frequencies. The signals can be indicated audibly by the receiver's loudspeaker as a spectrum of increased noise as the receiver is tuned through the spread. An operator can monitor the signals and turn off the lapping machine when the spread reaches a predetermined relation to a target frequency. The main limitation of this method is due to the fact that the signals are very weak, are shunted by the large capacitance between the lapping plates, and become progressively buried in electrical noise toward higher frequencies such that the upper practical frequency limits are about 15 MHz in planetary laps and 25 MHz in pin laps. The electrical noise originates from sources external and internal to the lapping machine. The lapping plate acts as an antenna for external signals such as radio transmissions and signals caused by neighboring electrical lines or apparatus. Most environmental signals could be shielded by means such as a Faraday cage, but this method is rarely used because it is cumbersome in practice and because of the additional noise internal to the machine. A major source for internal noise are metallic carriers, which are used in most planetary laps. The noise is due to electrical short circuits between the lapping plates by means of the carriers. At higher wafer frequencies these carriers are quite thin and will warp or buckle between the plates because of the lateral stresses exerted on them during lapping. This causes short circuits between the plates which are usually intermittent because of the randomly isolating effect of the slurry granules.
Automatic lapping control based on Method 4 is available but suffers from the described noise problem and is therefore rarely used at frequencies above a few MHz.
Method 5 is based on the injection of an electrical signal into at least one electrode imbedded in at least one of the lapping plates. If the frequency of the injected signal equals the resonance frequency of a wafer passing under an electrode, the impedance under the electrode shows a characteristic change which can be displayed by instrumentation such as an oscilloscope to indicate the occurrence of wafer resonance. An operator can monitor the wafer frequencies and terminate the lapping when they reach a predetermined relation to a target frequency. This method can be made less sensitive to external electrical noise than Method 4. However, it requires more expensive instrumentation and has other drawbacks which limit its usefulness and make it unsuitable for reliable automatic lapping control. This is explained in more detail in conjunction with the description of the invention.